In recent years, steel sheets made available for automotive components have been improved in strength in order to implement the crash safety and the fuel economy improvement. The automotive suspension components and frame components, and the like have also been improved in strength. However, the weight reduction of components requires the improvement of the fatigue strength as well as the static strength. Further, working into a complicated shape requires the compatibility with the workability (ductility or stretch flange formability).
It is known as follows: it is effective for the improvement of the workability that a DP steel including two kinds of microstructures having a large strength ratio is adopted; and further, as the method for improving the fatigue property of the DP steel, it is effective to harden the ferrite part low in strength and susceptible to a stress concentration. For example, Patent literature 1 describes as follows: in a DP steel including a main phase ferrite which has undergone precipitation hardening with a Ti or Nb carbide, and a hard second phase, the average ferrite particle size of the surface layer part to 20 μm is set at 5 μm or less. Patent literature 2 describes as follows: in a DP steel including a second phase including martensite/acicular ferrite/retained austenite, proeutectoid ferrite is subjected to precipitation hardening, thereby to improve the strength-workability-fatigue property.
For the hot-rolled steel sheets described in the Patent literatures 1 and 2, the holding/retention time at around 700 to 800° C. is set short, and Ti and Nb carbide particles are precipitated in a dispersed state in ferrite, thereby to precipitation harden the main phase ferrite. It is considered as follows: in the hot-rolled steel sheet, the precipitate precipitated finely in a dispersed state with holding/retention in a short time within the temperature range acts as an obstacle against the repeating motion of dislocation, thereby to improve the fatigue property. However, in the related-art technology, this cannot be said to produce a sufficient fatigue property improving effect.
Under such circumstances, with an aim to further improve the fatigue property of a DP steel, the present inventors conducted a close research and development on the precipitation hardening of ferrite in a DP steel. As a result, the following fact was found: when, in a DP steel, ferrite is hardened by precipitates of Ti, Nb, V, and the like, the holding/retention time within the temperature range is increased, thereby to appropriately coarsen the precipitates; this can produce a high fatigue property improving effect. Based on this finding, the following high-strength hot-rolled steel sheet was completed, and has already been filed for patent (see Patent literature 3).
The high-strength cold-rolled steel sheet proposed by the present inventors in Patent literature 3 (which will be hereinafter referred to as a “prior invention steel sheet”) is,
a high-strength hot-rolled steel sheet excellent in strength-elongation balance and fatigue property which includes, by mass %, C: more than 0.01% and 0.30% or less, Si: 0.1% or more and 2.0% or less, and Mn: 0.1% or more and 2.5% or less, and includes one, or two or more of V: 0.01% or more and 0.15% or less, Nb: 0.02% or more and 0.30% or less, and Ti: 0.01% or more and 0.15% or less so as to satisfy the following conditional expression (1), and the balance including Fe and inevitable impurities, and which has a microstructure having a ferrite fraction of 50% or more and 95% or less, and a hard second phase fraction including martensite+retained austenite of 5% or more and 50% or less, wherein the average particle size r of precipitates formed in the ferrite is 6 nm or more, and the average particle size r and the precipitate fraction f expressed by the following expression (2) satisfy the following conditional expression (3).C−12×(V/51+Nb/93+Ti/48)≧0.01  (1)f=(2.08Ti+1.69V+1.14Nb)/100  (2)r/f≦13000  (3)
Herein, each symbol of element in the expressions (1) and (2) means the mass % of the element.
The prior invention steel sheet is excellent in workability and fatigue property. However, the automotive components after working are often joined by welding to the car body, other members, or the like to be used. It is known as follows: in such a case, the heat affected zone (which will be also hereinafter referred to as “HAZ”) is more reduced in fatigue strength than the base material. For this reason, when automotive components are welded and joined to be used, a mere improvement of the fatigue property of the base material is not enough, and it is important to improve even the fatigue property of the HAZ. The prior invention steel sheet produces an excellent effect on the improvement of the fatigue property of the base material. However, there is a room for improvement of the fatigue property of the HAZ.
On the other hand, as the method for improving the fatigue property of the HAZ of the hot-rolled steel sheet, there is disclosed the following welding method: for welding, welding is performed after preheating the top of the welding line to 350 to 500° C.; as a result, HAZ is caused to include retained austenite, so that the fatigue property of the HAZ is improved (see Patent literature 4). However, this method requires the pre-heating operation before welding, and hence is unfavorably inferior in operability of the welding procedure.